This invention relates to a synchro motor type instrument, and more particularly to an indicator in which an automatic return member and a needle are directly connected to a rotor of a synchro receiver to thereby make the needle return to the predetermined OFF position when synchro signal is not applied by a synchro transmitter.
A typical connection diagram of an ordinary synchro motor system is shown in FIG. 1, wherein a synchro transmitter 2 and a synchro receiver 3 are mutually and electrically combined with the three-phase windings 2b1, 2b2, and 2b3 of a stator 2b 3 being connected to the corresponding three-phase windings 3b1, 3b2, and 3b3 of a stator 3b. On the other hand, single-phase windings 2a1 and 3a1 which are respectively wound on rotors 2a and 3a are connected in parallel to each other and are excited by single-phase AC power source E through brushes (not shown) and sliprings (not shown), whereby an induced voltage is generated on the three-phase windings 3b1-3b3 of the stator 3b of the synchro receiver 3. An AC signal is also present on the three-phase transmitter windings 2b1-2b3.
In the case when the rotational angle position of the synchro transmitter 2 is fully equal to that of the synchro receiver 3, the induced voltages of the synchro transmitter 2 and the synchro receiver 3 are equal, and accordingly electric current does not flow between the mutual three-phase windings 2b1-2b3 and 3b1-3b3. Consequently, torque is not generated in the synchro receiver 3. But, when the rotor 2a of the synchro transmitter 2 is rotated by a certain angle in response to a load, a difference is produced in the angle of rotation between the synchro transmitter 2 and the synchro receiver 3, both of the induced voltages becoming unbalanced. Accordingly, electric current flows between the three-phase windings, and the rotor 3a of the synchro receiver 3 follows the rotor 2a of the synchro transmitter 2 and rotates to the same rotational angle as the rotor 2a, the electric current generating such a torque as to return the rotor 3a to its synchronous position. By using this torque, the aforementioned system operates a needle to perform as a remote angle transmission.
Nowadays, although synchro motor type aero-instruments which work by receiving the above-mentioned synchro signal are widely used, such apparatus have the following defects when used for the design of a small-vertical type aero-instrument. That is, inasmuch as it is required that the width be small, as illustrated in FIG. 2, for the appearance of a small-vertical type instrument, an included synchro motor 10 must be a pancake-type synchro motor which is short in an axial direction so as to be suitable for the space in the housing. However, since it is difficult to modify the shape of a conventional type synchro motor to that of a pancake-type shape, synchro motor type small-vertical instruments have rarely been used.
Furthermore, techniques have been tried to mount a small synchro motor for aircraft to a vertical instrument. That is, referring to FIG. 3, the bracket motor of the small synchro motor 10c is clamped to the mounting plate 10d in the housing 10b with a pair of screws 10e. Then, when adjusting the needle 10f to the synchro zero-position, an operator takes off housing 10b and loosens screws 10e to move the indicator plate 10d so as to adjust the zero-position of the needle 10f. Thus, it was difficult to adjust the zero-position the needle from outside of the instrument housing.